Update project.json to remove redundant System.Runtime dependency.
Update project.json to remove redundant System.Runtime dependency.

- This package is pulled in transitively.

aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore